1. Field of the Invention
The present invention relates to electrophotographic copying.
More particularly, the invention relates to a method of, and an apparatus for, making electrophotographic copies of an original.
2. The Prior Art
In electrophotographic copiers a surface of a charge carrier has a uniform electrostatic charge applied to it. Thereafter, an image of an original is formed on this surface (usually projected onto it) by an appropriate optical system. This discharges the surface areas in accordance with the intensity of the radiation that reaches them and thus forms on the surface a latent electrostatic image.
To develop this image a dry, pulverulent developer material is applied to the surface to which it adheres electrostatically, in accordance with the latent electrostatic image, so as to form an actual visible image on the surface. This visible image is then transferred to a copy carrier, e.g. a sheet of paper or the like, by contacting the charge carrier surface with a copy carrier and transferring the developer particles to the copy carrier. Finally, the actual image which is now present on the copy carrier, must be fixed thereon to prevent the developer particles from rubbing off with a resultant destruction of the image.
This entire process is well known in the art and discussed in detail, e.g. in U.S. Pat. No. 3,062,109 of C. R. Mayo et al, to which reference may be had.
A proposal for eliminating the image transfer from the charge carrier to the copy carrier has been made in German Published Application No. (OS) 2,418,240. According to this proposal the latent electrostatic image is produced on a surface of a durable, inexpensive charge carrier of, e.g. Se, As.sub.2, Se.sub.3 or CdS. The surface is then contacted with a copy carrier and the image is thereafter developed directly on the copy carrier instead of on the charge carrier, so that no image transfer is required. The copy carrier itself is not required to be photoconductive.
Furthermore, the developer contacts only the copy carrier and not the charge carrier in this proposal. This makes it possible to use a liquid developer (i.e. a liquid carrier having toner particles suspended in it) or to employ electrophoretic development, both of which produce superior images and are best suited for making half-tone reproductions.
Theoretically, liquid developer could of course also be used with the conventional electrophotographic copiers described in the introductory paragraphs. However, this is not done because it has certain disadvantages. The transfer of a liquid-developed image from the charge carrier to the copy carrier is difficult and will, as a rule, result in a deterioration of the image. Moreover, the frequent contact of the developer liquid (and the substances dissolved therein) with the photoconductive surface of the charge carrier causes this surface in time to undergo changes which undesirably change its characteristics. Also, the drying and/or wiping of the photoconductive charge carrier surface, which must be carried out subsequent to developing of the latent image, cause a rapid wear of the surface.
All of these problems are avoided in the second proposal mentioned above, which thus constitutes an important advance in the art. However, it has been found that further improvements are desirable, inasmuch as the contacting of the charge carrier surface with the copy carrier tends to damage or at least weaken the latent electrostatic image which is present on the charge carrier surface.